1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device using a gettering technique. In particular, the present invention relates to a method of manufacturing a semiconductor device that uses a crystalline semiconductor film manufactured by adding a metallic element having a catalytic action in crystallizing a semiconductor film.
Note that, throughout this specification, the term semiconductor device indicates general devices capable of functioning by utilizing semiconductor properties. Electro-optical devices, semiconductor circuits, and electronic devices are all semiconductor devices.
2. Description of the Related Art
Thin film transistors (hereafter referred to as TFTs) are known as typical semiconductor elements that use a semiconductor film having a crystalline structure (hereafter referred to as crystalline semiconductor film). TFTs are in the spotlight as a technique of forming an integrated circuit on an insulating substrate such as glass, and devices such as liquid crystal display devices having integrated driver circuits are being put into practical use. In conventional techniques, crystalline semiconductor films are manufactured from an amorphous semiconductor film deposited by plasma CVD or reduced pressure CVD by using a heat treatment process or a laser annealing method (a technique in which a semiconductor film is crystallized by irradiation of laser light).
A crystalline semiconductor film thus manufactured is an aggregate of a plurality of crystal grains, and its crystal orientation is arranged in arbitrary directions. It is impossible to control the crystal orientation, and this consequently causes limitations in properties of the TFT. In solving this problem, Japanese Patent Application Laid-open No. Hei 7-183540 discloses a technique in which a metallic element having a catalytic action with respect to semiconductor film crystallization, such as nickel, is added and a crystalline semiconductor film is then manufactured. This not only has an effect of lowering a heating temperature required for crystallization, but it also becomes possible to increase the crystal orientation arrangement to become more unidirectional. If a TFT is formed by using this type of crystalline semiconductor film, then not only does it become possible to increase the electric field effect mobility, but a subthreshold coefficient (S value) also becomes smaller, and electrical properties increase significantly.
However, if a metallic element having a catalytic action for crystallization is added, the metallic element remains within the crystalline semiconductor film or on the surface of the film, and there are problems such as fluctuation in properties of elements obtained. Examples thereof include problems such as an increase in an off current in the TFT and its fluctuation between the individual elements. That is, the metallic elements that have a catalytic action for crystallization exist unnecessarily after the crystalline semiconductor film is formed.
Gettering using phosphorous is an effective and often used method for removing this type of metallic element from specified regions of the crystalline semiconductor film. For example, it is possible to easily remove the metallic elements from a channel forming region by performing a heat treatment process at a temperature of 450 to 700° C. by adding phosphorous to a source or drain region of the TFT.
Phosphorous is injected into the crystalline semiconductor film by an ion doping method (this indicates a method of dissociating PH3 or the like by a plasma, accelerating the ions by using an electric field, and injecting the ions into the semiconductor film; the ion doping method is basically a method in which separation of mass of ions is not performed). The concentration of phosphorous necessary for gettering is equal to or greater than 1×1020/cm3. Adding phosphorous by ion doping can cause a crystalline semiconductor film to take on amorphous qualities, and the increase in the phosphorous concentration hinders recrystallization during a later annealing process, thus becoming a problem. Further, the addition of a high concentration of phosphorous causes an increase in the required amount of processing time for doping, and throughput of the doping process step is decreased, thus becoming a problem.